The present invention relates to an electrolysis cell for the electrolytic removal of an electrochemically convertible impurity from gases as well as to a method of purifying gases using this electrolysis cell.
Gaseous pollutants or impurities in gases such as e.g. Cl.sub.2, SO.sub.2, NO.sub.x and mercaptans can be removed by a wet-chemical wash with a wash solution. Absorption apparatuses such as e.g. packed columns are used for such a purification. In order to obtain a good purifying action the absorbed gaseous component must be rapidly converted. A disadvantage is the necessity of having to use certain chemicals. As a result, a waste-water disposal problem frequently arises on account of the reaction product formed in the wash solution and stemming from a waste gas problem.
Methods are even known for the electrochemical purification of waste gases. In the so-called "outer-cell" methods the gaseous component of a waste gas to be stripped is first absorbed in an absorption column in a wash solution; then the wash solution containing the polluting component is cathodically reduced or anodically oxidized in a connected electrolysis cell. This arrangement requires two different devices, namely one for the absorption and one for the electrolysis. The so-called "inner-cell" methods are more advantageous, in which absorption and electrochemical conversion take place in a reactor, namely in an electrolysis cell, and because the concentration of pollutants is always kept low by electrochemical conversion. In the so-called "indirect" electrolysis processes the oxidizing or reducing agent used in a wet-chemical waste-gas treatment is regenerated by electrolysis of the wash solution used.
An "inner-cell" method suitable for the oxidative or reductive electrolytic purification of waste gases is taught by G. Kreysa et al. in Ger. Chem. Eng. 6 (1983) 325-336: The gas to be purified is brought in contact with an electrolytically conductive wash liquid in an absorption column designed as a fixed-bed electrode. The absorption column contains a packing of conductive particles. The gas to be purified and the wash liquid can be conducted through the column in a cocurrent flow or in countercurrent flow. In both instances a stable two-phase mixture (gas/liquid) should flow through the column. The column should always be flooded and separation of the liquid flow should be avoided. Kreysa et al. show that under the indicated conditions a waste gas containing 5000 ppm chlorine can be stripped using a K.sub.2 SO.sub.4 catholyte solution and a current of 15 A to 50 ppm chlorine. The method evaluated previously as well as various embodiments of electrolysis cells suitable for this purpose also follow from DE-OS 29 01 577. Such a cell comprises electrode compartments separated by a diaphragm or membrane and with the associated electrodes in which at least one electrode is designed as a fixed-bed electrode. Also, several anodes and cathodes can be combined like a filter press. Several counterelectrodes with counterelectrode spaces and membranes can also be arranged within a larger fixed-bed electrode.
A disadvantage of the method of Kreysa et al. is the high residual content of impurities in the purified gas. In the case of chlorine the residual content is approximately a factor of ten above the limit value of 5 ppm admissible e.g. in Germany.
According to the not yet published DE patent application 195 31 707.6 better degrees of purification can be achieved on the basis of the previously evaluated method of Kreysa if the fixed-bed electrode is operated as a trickle-bed reactor. This procedure leads to lower investment costs than that of the previously known method with flooded reactor and in a considerably lesser residual content of impurities.
A disadvantage of the previously known method is the fact that the apparatus comprising an electrolysis cell requires two liquid circuits, namely a catholyte circuit and an anolyte circuit, as a result of which the device is rendered complicated and trouble-prone.
An object of the present invention therefore is to provide a method which permits the electrochemical purification of gases while using a simpler electrolysis cell and consuming less energy.